The present invention relates to a method of manufacturing an erasable programmable read-only memory (EPROM) device in which stored information can be erased by ultraviolet radiation.
EPROM IC devices have in recent years found wide uses in various electronic digital apparatuses.
Since information can be written in the EPROM and stored information can be erased from the EPROM by radiating ultraviolet rays, the user can easily vary a program stored in the EPROM.
The EPROM is composed of floating-gate MOS transistors capable of nonvolatile storage of information. U.S. Pat. No. 3,660,819 discloses in detail a floating-gate MOS transistor.
As shown in FIG. 1 of the accompanying drawings, an EPROM IC chip has conventionally been housed in a ceramic package having a window. Generally, the ceramic package comprises, as shown in FIG. 2, an alumina base plate 1, an IC lead frame having a mounting region 6 in which lead pins 3 and an IC chip 7 are connected, and an alumina base plate 2 having a window 11 for passing ultraviolet rays therethrough. These components of the package are sealed by a mass 4 of glass having a low melting point. The surface of the mounting region 6 is coated with a plated layer of gold.
The ceramic package for EPROM ICs has been disadvantageous in that a manufacturing process therefor has many steps resulting in an increased cost of the IC devices. Where the ceramic package is employed, pad electrodes of the IC chip and the lead pins 3 cannot be interconnected by gold wires since the melting point of sealing glass 4 is in the range of from 400.degree. to 500.degree. C. The pad electrodes of the IC chip and the lead pins 3 are interconnected in many cases by aluminum wires, which however tend to react with gold and becomes brittle. Therefore, as shown in FIG. 3, in order to ground the pad electrode 12 of the IC chip 7, it has been necessary for an aluminum wire 10 to be connected by way of the small silicon piece (ground dice) 9 on which an aluminum layer 8 is formed.
The wire bonding process using aluminum wires has a drawback in that the rate of bonding is slower than that in the wire bonding process using gold wires.
In case a ceramic package is employed for an EPROM IC, the package is likely to be broken by a shock. Where a number of such packages are mounted on a printed-circuit board, the resulting construction is highly heavy and cannot be handled with ease.
To solve the foregoing problems, it is proposed to encapsulate an IC chip in a plastic-molded package. For example, U.S. Pat. No. 3,611,061 discloses the techonology for molding an IC chip in a plastic package. However, if the disclosed technology is to be applied to an EPROM IC chip, the necessity of a window for passing ultraviolet rays therethrough has made it difficult to encapsulate the EPROM IC chip in a plastic-molded package.